An air conditioning system utilizing a central thermal source unit is widely used; which uses, as cold heat, cold/chilled water supplied by a chilled water circulation system.
In the foregoing air conditioning system, a primary return header and a primary feed header are connected to a plurality of thermal sources that supply chilled water, and the chilled water is fed from the primary return header, via the thermal sources, to the primary feed header by a primary pump. Additionally, a secondary feed header and a secondary return header are connected to a plurality of AHUs (Air Handling Units) that cool air by use of the chilled water. Disposed between the primary feed header and secondary feed header is a secondary pump that feeds the chilled water from the primary feed header to the secondary feed header. As an example of the secondary pump, one that is inverter-controlled is known. Connections are made so that the chilled water flows into the primary return header from the secondary return header.
As a method for controlling the flow rate of chilled water circulating in the air conditioning system, a method for controlling the flow rate of chilled water by performing control such that the pressure of chilled water discharged from the foregoing secondary pump is made equal to a specified set value is known. As described in Patent Documents 1, 2, another method is also known, in which the flow rate of chilled water fed out from a secondary pump is controlled such that a main-pipe temperature difference, which is the difference between the feed temperature, i.e., the temperature of chilled water fed out from a secondary feed header, and the return temperature, i.e., the temperature of the chilled water flowing into a secondary return header, is equal to a specified value.
In the techniques described in the Patent Documents 1, 2, only the flow rate of chilled water required in AHUs is provided. Accordingly, power for conveying chilled water can be reduced, that is, power consumed by the secondary pump can be reduced. Additionally, even if thermal load in the AHUs drops, the flow rate of chilled water is controlled such that the difference between the feed temperature and return temperature is maintained at the specified value. In other words, even if the return temperature decreases, control is performed so as to increase the return temperature. Accordingly, the operating efficiency of thermal sources can be maintained. Additionally, a sensor required for the foregoing control is a temperature sensor only. Since the need for a pressure sensor is therefore obviated, manufacturing costs can be reduced.
Patent Document 3 discloses control for increasing, on the basis of an alarm output from a specific AHU, the flow rate of chilled water fed out from a secondary pump. As an example of the alarm, one indicating that thermal load in the specific AHU has increased can be given. This control can inhibit an increase in room temperature or the like, resulting from insufficient capacity of the AHU.